Siebs' law

Siebs' law is a Proto-Indo-European (PIE) phonological rule named after the German linguist Theodor Siebs. According to this law, if an s-mobile is added to a root that starts with a voiced or aspirated stop, that stop is devoiced. Compare:

PIE *bʰṛHg- > Latin fragor,

but *s-bʰṛHg- > PIE *spʰṛHg- > Sanskrit sphūrjati.

Discussion

Siebs proposed this law in the Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen, as Anlautstudien (Berlin 1904: 37.277-324). Oswald Szemerenyi has rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan zdī from PIE *s-dʰi "be!" as counterproof (Szemerenyi 1999: 144). However, the PIE form is more accurately reconstructed as *h₁s-dʰí and thus Siebs' law appears to demand that the sibilant and aspirated stop are both adjacent and tautosyllabic, something which is known to only occur in word-initial position in Proto-Indo-European anyway.

References

• N. E. Collinge (1985). The Laws of Indo-European. John Benjamins Publishing Co. ISBN 978-0915027750. 
• Oswald Szemerenyi (1999). Introduction to Indo-European Linguistics. Oxford University Press. ISBN 978-0198238706.